1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
The coverage area of a wireless communication system is typically divided into a number of cells, which may be grouped into one or more networks. Mobile units located in each cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with a base station associated with the cell. The mobile units may include devices such as mobile telephones, personal data assistants, smart phones, Global Positioning System devices, wireless network interface cards, desktop or laptop computers, and the like. As a mobile unit moves between cells in the wireless communication system, the mobile unit may periodically provide location update messages that inform the wireless communication system of the mobile unit's current location. The wireless communication system may use the information in the location update messages to locate the mobile unit (e.g., by paging the mobile unit) and then direct information to the mobile unit via a base station indicated when the mobile unit responds to the page.
In some activity states, such as the idle or dormant mode, the mobile unit may not send location update messages even though it may continue to move through the cells in the wireless communication system, until some condition is met (e.g., when the mobile unit crosses the boundary of the tracking area associated with the last location update message, a new location update with the new tracking area is sent). Accordingly, the wireless communication system may not know the exact cell that includes the mobile unit when information becomes available for delivery to the mobile unit. The wireless communication system may attempt to reach the mobile unit by sending paging messages over a plurality of cells belonging to a paging area determined by the network based on the information it has about the last known mobile unit location. For example, the wireless communication system may attempt to reach the mobile unit by sending paging messages to the cells belonging to the last known tracking area.
The paging messages contain information that indicates to the mobile unit that information is available for transmission to the mobile unit. If the mobile unit receives the paging message, it may provide a paging response via a base station that provides wireless connectivity to the cell that includes the base station. The paging response typically indicates that the mobile unit is available to receive the information and also provides information indicating how to route the information to the mobile unit.
Both the paging messages and the location update messages represent system overhead. Accordingly, the wireless communication system is generally designed to meet two conflicting objectives: reducing the overhead from the paging load and reducing the number of location update messages transmitted by the mobile unit. The paging load is typically minimized when the location of the mobile unit is known with relatively high granularity so that each paging message can be transmitted to a relatively smaller number of cells. However, increasing the granularity of the location of the mobile unit requires transmitting a larger number of location update messages. In contrast, reducing the number of location update messages transmitted by the mobile unit may reduce the granularity of the location information used by the wireless communication system to locate the mobile unit, which typically results in each paging message being transmitted to a relatively large number of cells.
The conventional solution to this problem is to define tracking areas that include the cells serviced by a plurality of base stations. The mobile units may then transmit location update messages when they cross from one tracking area to another tracking area and the wireless communication system may begin the paging process by providing paging messages via the base stations in the tracking area indicated by the most recently received location update message. For example, the geographic area served by the wireless communication system may be divided up into multiple tracking areas that encompass the cells serviced by groups of 10 base stations. Mobile units in the wireless communication system may then provide location updates when they cross a cell boundary between the groups of 10 base stations and the wireless communication system may provide paging messages via the groups of 10 base stations in the tracking areas.
Conventional tracking areas may be static, i.e., the association of tracking areas to base stations remains constant over time, or dynamic, i.e., the wireless communication system may modify the tracking areas associated with a mobile unit. For example, an entity, such as a radio network controller, in a wireless communication system that implements dynamic modification of the tracking areas may determine a distance that the mobile unit has traveled between successive location update messages. If the mobile unit has moved a relatively large distance, the radio network controller may increase the size of the tracking area associated with the mobile unit to include a larger number of base stations. Conversely, if the mobile unit has moved a relatively small distance, the radio network controller may decrease the size of the tracking area. Conventional wireless communication systems may also dynamically adjust the size of tracking areas based on a velocity of the mobile unit.
Implementing static tracking areas and accounting for movement of the mobile units through these tracking areas using entities in the wireless communication system, e.g., in a radio network controller, increases the computational load in the wireless communication system. The computational load may be further increased if the tracking areas are dynamically assigned by the wireless communication system, at least in part because the algorithms for assigning and/or modifying tracking areas associated with each mobile unit are computationally much more complex than the algorithms used to implement static tracking areas. For example, each radio network controller may need to acquire, store, and manipulate information indicating at least the current and previous locations of each mobile unit served by the radio network controller, as well as the size and/or constituent base stations of the tracking areas associated with each mobile unit served by the radio network controller.
Mobile units may also be able to support multiple modes that permit the mobile units to access different wireless communication technologies. For example, a first technology may be used to provide wireless connectivity over a first area and a second technology (e.g., a new or updated technology) may be used to provide wireless connectivity over a second area that overlaps with a portion of the first area. Thus, when the mobile unit moves it may encounter areas where the environmental and/or radio conditions cause the mobile unit to move between the different technologies, which may cause the mobile unit to transmit location update messages. The proliferation of different wireless communication technologies, as well as the increasing market demand for access to these technologies, is expected to result in an increase in the number of multimode mobile units that can access multiple technologies. Consumers will also expect the multimode mobile units to move seamlessly across multiple technologies.